How is it possible to place XL breast implants under the muscle?

Based upon your anatomy, there is a limit to the maximum size breast implant that can be placed submuscular. It is unreasonable to think that the largest and most compliant tissues could accommodate that. 

Large implants can be placed under the muscle in some, but not all women. However, large implants, because of the weight, tend to sag and may fall out from under the muscle. This will then encourage even more sagging. You might consider going down in size. Over time, you will probably have fewer issues with implants that better fit your frame and don't overstretch your tissue. Big breasts sitting on your lap are not pretty.

Without examing you , it is difficult to comment on how large an implant can be placed in either a submuscular or subglandular position. Certainly , in some patients this can readily be accomplished. In my practice , I do not recommend placement of implants on top of the muscle for a number of reasons. I also try to dissuade patients from having overly large implants placed at all. Most patients with extremely large implants for the available tissue coverage will experience increased problems with rippling, contour deformities , implant migration, and capsule contracture. When these problems occur , they can be quite challenging to correct with a good cosmetic result.